Direction finding



a-Y 5, 1947- C. w. HANSELL DIRECTION FINDING Filed Aug. l. 1942 ence mil INVENTOR BY 4Z ArroRNEY Patented May 6, 1947 DIRECTION FINDING Clarence W. Hansell, Port signor to Radio Corp corporation of Delawar Jefferson; N. Y., as-

oration of America, a e

Application August 1, 1942, Serial No. 453,252

This application concerns a new and improved method of and means for locating remote wave energy sources or points at which wave energy is reiiected from remote or local wave energy sources. The wave energy sources may be of ultra-high-frequency, radio-frequency, sound waves or infrared rays. To simplify the disclosure they have been referred to herein as Wave energy or radiant energy.

`1in this new method and means I make use of a phenomenon disclosed in detail in my U. S. application #389,161, led April 18, 1941, Patent No. 2,388,052 issued October 30, 1945. As disclosed in said `application, when two currents of diierent frequency ,fl and f2 but of substantially equal amplitudes Vi and V2 are passed through an amplitude limiter, the resulting current coming from the limiter is a current at a frequency f3 approximately half way between the frequencies of the two input currents, but this constant frequency output current suiers a substantially instantanteous phase reversal each time the two input current components are passing through the condition oi opposing phase. This is equivalent to saying that the output current from the limiter is at a frequency about half way between the two input frequencies except for very great momentary frequency shifts which take place at the in stant of opposing phase of the two input currents, that is, once per cycle of the difference frequency as shown in Figure 3 of my above identified application.

If the two input currents are slightly diierent in amplitude, the momentary large frequency shift will be in a direction from the said intermediate value of frequency toward and through the frequency of the stronger input current. If the relative magnitude of the two input currents is reversed, `the direction of the momentary frequency shift is reversed.` Consequently, an extremely small change in relative amplitudes of the two input currents around the condition or" equality will reverse the direction of the large peak frequency shifts.

An amplitude limiter as defined for the purposes of this patent application is any device which removes substantially all variation in strength or amplitude of alternating currents passed through it but leaves substantially al1 variations in phase or frequency of the currents undisturbed, within a required frequency band. A number of devices capable of use as amplitude limiters are already well known in the art and it appears likely that many other devices have been or will be devised to accomplish limiting.

If two currents of diiierent frequencies, the respective relative strengths of which are proportional to the positioning or orientation of two directive pick-up devices with respect to a radiant energy source (direct or reiiected) are applied to an amplitude limiter, a resultant current is ob- 3 Claims. (Cl. Z50-11) tained of a frequency intermediate the frequencies of the two currents except for sharp peaks which extend away from the weaker current toward and through the stronger current so that the average frequency of the resultant is equal to the frequency of the stronger current. These sharp peaks occur when the currents oppose in phase. Accordingly, there is provided a resultant current with large frequency deviations which are in the direction from the mean or intermediate irequency toward and through the stronger current. This resultant is then passed through a frequency modulation detector and the peaks detected to provide an indication of which current is the stronger to thereby provide an indication of the positions of the two pick-up devices relative to the wave source and finally an indication of the wave source position. Thus, extremely small differences in the strength of the two currents can be detected, and an extremely accurate indication of the direction of the source can be provided.

In describing my invention in detail reference will be made to the attached drawings where, in 1s'igure l, I have shown an embodiment of a radiant energy source direction locating system arn ranged in accordance with my invention and uti` lizing the phenomenon which takes place in an amplitude limiter when two currents of nearly equal strength are applied to it. In the iigure is shown a direction finder system applicable to radio navigation, collision prevention and object nding. Obviously, the invention may be put to many other uses.

In Figure 2 I have shown the directivity pattern oi the pick-up devices used in the system of Figure 1` In Figure 1, A and B are directional pick-up devices of appropriate type and, as shown, are mounted for movement or orientation so that their response characteristics can be altered. l2,

and 4 are sources of oscillations coupled respec` 8 and I0. The heterodyne detec-v. supply the two currents .fi and ,f2 y to an amplier and limiter i6, Wherein'the plie-s. nomenon described before takes place so that a resultant current f3 is fed to the frequency modutively to mixers tors 8 and l erally at 26.

TheV system comprises two directional an.

tennas, A and rent.

nearly equal in magnitude; the wave for-m of4` 3 equality of the two currents, the antennas may be utilized for very sensitive indications of direction from the antennas to the source.

In making a directional observation the an;-` tenna may be rotated physically until they pick` up equal currents or, over a range of directions, the currents may be unequally attenuated or am'- pliied to make the resultantcurrents equal-,gin which case, the difference iri attenuation or amr plication of the currents will provide an indication oi direction. In Figure-r, is is assumed that the antennas are physically rotated and the direction to a. source of radio waves willbc indi` cated by the position vat which the antennas deliver equal currents--` In the system of Figure 1, currents from the two antennasare delivered to-two similar heterodynedetectors B and Hi, outputs from which are combined in V a single output circuit and delivered through an amplitude limiter Hi' to a frequency modulation detector IS;

The two. heterodyne detectors are supplied with locally produced heterodyning currents from I2 and' I4 which are somewhat diierent in' frequency so that the .two detector output currents have different frequencies, fl and f2, and beat together inthe i amplitude limiter.

The current, f3-, in the output from the limiter will be a newcurrent with an average, or carrier, frequency equal to the frequency of the stronger of the currents entering the limiter and this currentwill be phase modulated by the weaker cur-v W-hen the two limiter inputcurrentsare phase modulation and the correspond-ing wave formv of frequency modulation will be greatly distorted with-relativelyhigh peaks in one direction; Furthermore, as previously explained, the direction o1- frequency swing at the pea-k; or the polarity of the modulation peak; willreversel changes from a lesser to Ithe strength of the other abruptly as one current a` greater strength than current.

Output from the amplitude limiter, with itsdistorted` wave form offrequencyy modulation produced bythe interaction offtheitwocurrents in the limiter is applied to a frequencymodulation detector, I8., the output of 'which reproduces the distorted wave form: of frequency modulation.

The distorted wave form of current maybe observed on a, cathode" ray oscilloscopeY as; previouslyexplained-in connection withFigure. 5 of thcabove-mentioned application, or it may be utilized toi produce atI directcurrent which re-l` versesin direction as-thepolarity'of the modulaition peakr reverses; by meansofthey` arrangement shown in Figure 1.

The indicator of Figure l consistsof twofrecti# ers of anysimilar type delivering rectififedfdie rectcurrentAV pulses to a; common resistance-cone denser rectifier bias or load` Q of the unsymmetrically peaked wave: formi offrpoa tential delivered to the rectifiers onerectifieris supplied with much greater peak forwar'dpotential than; the other and-therefore supplies allor most-of` the rectifiecrcurrent. Il?`v thepolarityof the` peaks--is-reversed, the currentis transferred from one rectifier tothe other. This transfer'off current-from one rectifier to the other is utilizedthrough coupling resistances 26: to supply input'- toK a reverse current indicating instrument.

In taking a bearing the antennas are-rotated until the reverse current instrument shows-a defiection in one direction or circuiti-24'; Because'- 1 the other toshowlthat tnegistant source of radio waves f is-withinV the.-

- of a first frequency,

beam of one of the antennas. It is further rotated until the polarity of t-he' current suddenly reverses- Then the direction of the' distant radio waves is indicated, with an extreme precision of indication, by the direction in which the indicator deflection reverses.

By this means extremely precise indications of direction are obtainable with each of the antennas set notv'ery far from its maximum of reception so that noise has a small effect upon the precisionoi-the directional indication. This is in marked contrast to the usual method of direction finding in which a minimum of antenna pick-up from the distant radio wave source is used to indicate direction and in which noise has a maximum possibility to disturb the precision of directional indication.

I claim-z y 4 l-. In apparatusof the class describedatleast` two directional pick-up devicesin the field ofv a source of radiant energy, connections to one of said pick-up devicesfor producing wave energy connections to the other of said pick-up devices Jfor producing wave energy of a second frequency, the amplitudes of said wave energies depending on the positioning of said pick-up devices with respect to said source, an amplitude limiter, connectionsfor impressing bot-h of said wave energies onf said amplitude' limiter, awave length modulation detector cou-- pled to said amplitude limiter, and an indicator coupled to' said detector. l

2. The method of determining the direction of arrival of al radio wave; includingthe steps oideriving iroin saidwave-two voltages of different frequencies which are differently related in amplitude to theY angie oiarrival of said wave withrespect toy an angularly adjustable reference line,y combining said voltagesv voltage-having?variations inv amplitude and-` variations in instantaneous frequency which aref a: function of the difference in amplitude between said two voltages,` limiting saidresultantvoltage toY remove therefrom said variations inA amplitude;

` detecting said1 variations in instantaneousfre quency to produce an alternating current-wave4 having positive and negative peaks, and adjusting the ang-le of said' reference line to cause-oppositepeaks of-said alternating current Wave to-be-equal in magnitude.

3.- A directiorrnderincluding directional pick: upmeans, y means for deriving therefrom in re"- sponsetothe arrivaiofenergy twor voltagesor different frequencies having amplitudes depende ing; uponthe angular position; oi saidpickupmeans with respect to the line of arrival-off said energy', means -for combining: said voltagesto-"pr duce a resultant voltage varying in instantaneousfrequencyv in response to the difference in-amplitude between said-two voltages; and niea-ns fordetectingsaid variations inf frequency.`

CLARENCE W.

REFERENCES errer) The following' references' areA ofi record inthe;v

file? of' this-patent:

UNITED STATES PATENTS to produce a'1 resultant- 

